Spinal Cord, Peripheral Nervous System, and Reflex Arcs

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Spinal Cord, Peripheral Nervous System, and
Reflex Arcs

• Chapter 12 - 13

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Characteristics

• 17 long ¾ thick
• 2-way conduction pathway to and from brain
• Major reflex center
• Extends from foramen magnum to L1
• Posterior to L1 is cauda equina

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• Consists of gray matter (unmyelinated nerve
  cells) and white matter

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Gray Matter

• Looks like an H in cross section
• Contains
• posterior (dorsal) horns - association neurons
• Anterior (ventral) horns - cell bodies of somatic
  motor neurons

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• Amount of gray matter reflects amount of skeletal
  muscle innervated at that level

Dorsal root
Ventral root
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Nerve Roots

• Dorsal nerve root carries incoming messages
  (sensory)
• Ventral nerve root carries outgoing messages
  (motor)

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• Dorsal and ventral roots join to form 31 spinal
  nerves
• Poliomyelitis gray matter inflammation
  viral disease which destroys the ventral horn of
  the spinal cord virus is contracted through
  contaminated water Salk and Sabin vaccines
  prevent infection

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White Matter

• Nerve tracts of myelinated and unmyelinated
  fibers running in 3 directions
• Up (ascending sensory)
• Down (descending motor)
• side to side

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Pathology

• Paralysis loss of motor function
• Parathesias loss of sensation
• Paraplegia loss of lower limb function
• Quadraplegia loss of all limb function

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• Hemiplegia brain disorder resulting in loss of
  function on one side
• Flaccid paralysis loss of muscle tone due to
  damage of ventral horn or root
• Spastic paralysis damage to brain neurons
  muscles are stimulated by spinal nerves

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Peripheral Nervous System

• Chapter 13

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Cranial Nerves

• 12 pair only one of which extends down into the
  body
• Usually named for what they innervate
• Given Roman numerals in the order in which they
  are located
• O-O-O-T-T-A-F-V-G-V-A-H (structural)

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• Oh-Oh-Oh-To-Touch-And-Feel-Very-Good-Velvet-AH!
• I Olfactory
• II Optic
• III Oculomotor
• IV Trochlear external eye muscle

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• V Trigeminal sensory from eye, nose, cheek,
  mouth, scalp
• VI abducens external eye muscle
• VII facial skeletal muscles of face, tear
  ducts, taste buds
• VIII vestibulochochlear equilibrium, hearing
• IX Glossopharyngeal swallowing, gag reflex,
  taste

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• X Vagus heart, lungs, abdominal viscera,
  taste, swallowing
• XI Accessory larynx, pharynx, soft palate,
  neck and head movement (shrugging)
• XII Hypoglossal tongue movement

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• Cranial nerves can be sensory, motor, or mixed
  (both)
• Some-Say-Marilyn-Monroe-But-My-Brother-Says-Bridg
  et-Bardot-Mmmm-Mmmm

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Spinal Nerves

• 31 pairs all are mixed nerves
• 8 cervical
• 12 thoracic
• 5 lumbar
• 1 coccygeal

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• Spinal nerves leave the spinal cord, branch and
  then converge again to form a complicated network
  called a plexus
• Allows muscles to receive its nerve supply from
  more than one spinal nerve root

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• Therefore a damaged nerve does not lead to total
  motor or sensory loss in the limb region served

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• There are several major plexi
• Cervical neck phrenic nerve (diaphragm) is
  here
• Brachial upper limbs radial nerve is the
  largest

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• Lumbar abdominal wall and thigh femoral nerve
  is largest
• Sacral thigh and lower leg sciatic nerve
  (largest and longest nerve in body)

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Reflexes

• Reflexes are rapid, predictable motor responses
  to stimuli unlearned, involuntary,
  unpremeditated 2 kinds
• Somatic skeletal muscle
• Autonomic glands and involuntary muscle

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• Reflex arc contains 5 elements
• Receptor (thermo, chemo, mechano, noci)
• Sensory neuron
• Integration center spinal cord
• Motor neuron
• Effector (muscle or gland)

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• Types of reflexes
• Stretch (deep tendon) reflex coordinates muscle
  control of paired muscles (bicep/tricep)
  prevents over stretching, muscle tone
• Withdrawl reflex initiated by actual or
  perceived painful event prick finger, hot stove,
  punch to abdomen, etc.

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• Crossed extensor reflex complex helps to
  maintain balance
• Superficial reflex initiated by gentle touch
  Babinski sign
• Pupillary reflex - protective